Description

This PR refactors the application top bar by extracting shared logic and
layout into a reusable AppBarContent component and consolidating all
user-related interactions into UserMenu.

Both the admin and main UI now use the same app bar foundation,
reducing duplication and improving consistency across the application.

What changed

• Replaced duplicated v-app-bar implementations in both admin and main UI with a new reusable <AppBarContent> component.
• Extracted user-related UI and logic into a standalone <UserMenu> component.
• Introduced slot-based layout in AppBarContent (left and right slots) for flexible composition.
• Unified app bar behavior (menu toggle, support link, user menu, dark mode toggle) across interfaces.
• Updated tests and snapshots to reflect new component structure.
• Improved responsive handling in Namespace component by truncating long namespace names based on screen size.

Reasoning

• Eliminates duplicated app bar logic between admin and main UI
• Centralizes common behaviors (menu toggle, support action)
• Improves maintainability and consistency for future UI changes
• Makes app bar features easier to extend without touching layouts

##How to test

1. Run the admin and main UI applications
2. Verify the following behaviors:
   • Menu toggle works on mobile view
   • Support button opens the ShellHub help page
   • User menu actions behave as before
   • Dark mode toggle works correctly
3. Run unit tests and confirm snapshots pass